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From Carbon Dioxide to Methane: Homogeneous Reduction of CarbonDioxide with Hydrosilanes Catalyzed by Zirconium-Borane Complexes

Tsukasa Matsuo* and Hiroyuki Kawaguchi*

Coordination Chemistry Laboratories, Institute for Molecular Science, Myodaiji, Okazaki 444-8787, Japan

Received July 4, 2006; E-mail: tmatsu@ims.ac.jp; hkawa@ims.ac.jp

Carbon dioxide is the stable carbon end product of metabolism

and other combustions, and it is an abundant yet low-value carbon

source. This molecule would be very valuable as a renewable source

if it were to be effectively transformed into reduced organic

compounds under mild conditions.1 However, thermodynamic

stability of CO2has prevented its utilization in industrial chemical

processes, and thus this represents a continuing scientific challenge.

Here we show that CO2 is catalytically converted into methane

(CH4) and siloxanes via bis(silyl)acetals with a mixture of a

zirconium benzyl phenoxide complex and tris(pentafluorophenyl)-

borane (B(C6F5)3). In addition, an appropriate choice of hydrosilane

may lead to selective formation of the initial acetal product andisolation of polysiloxane materials.

The overall reaction stoichiometry described here proceeds as

illustrated in eq 1. We have studied early transition metal complexes

having phenoxide-based multidentate ligands to understand their

reactivity toward small molecules such as dinitrogen and carbon

monoxide.2 As part of these investigations, we have begun to study

activation of carbon dioxide. Synthetic systems capable of reducing

CO2to CH4have been elusive, despite the wealth for precedent of

metal complexes that undergo the conversion of CO2into a variety

of organic products.3-5 Hydrosilation of the carbon-oxygen double

bond is a mild method for reduction of carbonyl functions of organic

compounds.6 Since the reaction is exothermic, the use of hydrosi-

lanes in the reduction of CO2 is very attractive. For example, the

reactions of CO2with hydrosilanes mediated by late transition metal

complexes have produced silyl formate and methoxide.7,8

In a first set of experiments, we studied the catalytic activity of

cationic zirconium benzyl complexes bearing phenoxide ligands

(Chart 1) in the course of reducing CO2with PhMe2SiH as the test

substrate. The catalysts used in this study were synthesized by

treatment of the dibenzyl complexes with B(C6F5)3 in toluene.2,9

Reaction of (L3)Zr(CH2Ph)2 with [Ph3C][B(C6F5)4] in toluene

solution produced Ph3CCH2Ph and [(L3)Zr(CH2Ph)][B(C6F5)4].

Since improved reactivity and yield were achieved through in situ

generation of cationic species, subsequent experiments were

performed by premixing the dibenzyl complexes and boranes prior

to the addition of hydrosilanes and CO2.

The results of the catalytic CO2 reduction with PhMe2SiH are

depicted in Table 1. The reaction proceeded exothermically to form

(PhMe2Si)2O. Performing the analogous reaction in benzene-d6 in

a NMR tube revealed the release of CH 4 as the byproduct. The

resonance due to CH4 was observed as singlet at 0.15 ppm in the1H NMR spectrum. Mono- and bisphenoxide ligands L1 and L2

gave zirconium complexes that preformed with low activity (entries

1 and 2) relative to the tridentate ligand L3. It is obvious from

entries 4 and 5 that the combination of a zirconium complex with

B(C6F5)3 is responsible for this result. Thus, neither zirconium

cationic species nor B(C6F5)3alone provided an active catalyst. The

enhanced reactivity associated with the tridentate-phenoxide ligand

made (L3)Zr(CH2Ph)2 attractive for further study.

To obtain insight into the intervening processes in the reaction,

isotopically enriched 13CO2 (99 atom % 13C) was admitted into a

resealable NMR tube containing a solution of (L3)Zr(CH2Ph)2,

B(C6F5), and Et3SiH in benzene-d6at room temperature. When Et3-

SiH is used as a hydrosilane, the reaction requires approximately

one week for completion and is easily monitored by NMR

spectroscopy (Figure 1). The reaction proceeded cleanly, during

which time 13CO2 and Et3SiH were fully consumed. Monitoring

the reaction by 13C{1H}NMR spectroscopy indicates the presence

of bis(silyl)acetal13CH2(OSiEt3)2as a detectable intermediate. The

resonance at 84.5 ppm due to 13CH2(OSiEt3)2grew to a maximum

relative intensity over a period of about 7 h and then decreased as

that at -4.4 ppm due to 13CH4. It then continued to grow until,

after about 1 week, it was the only resonance attributable to a 13C-

labeled product. Additionally, in the absence of proton decoupling,

the resonances due to 13CH2(OSiEt3)2and13CH4split into a triplet

and a quintet with a coupling constant of 161.5 and 125.6 Hz,

respectively. This observation unambiguously confirms that the

carbon atom of CH4 originates from CO2, and the source of its

hydrogen atoms is added Et3SiH.

The scope of reduction of CO2 by various hydrosilanes was

examined with 3/B(C6F5)3 as the catalyst (Table 2). The six

hydrosilanes examined were shown to be effective for reduction

of CO2 to CH4, and the 3/B(C6F5)3 catalyst is sensitive to steric

hindrance around the substrate Si-H bond. For example, triethyl-

silane reacted more slowly than diethylmethylsilane (entries 1 and

2), while bulkier triphenylsilane did react but required more catalyst

and prolonged reaction time to give a mixture of (Ph3SiO)2CH2and (Ph3Si)2O (entry 4). Primary and secondary silanes can also

be employed. The reaction with diethylsilane produced a mixture

CO2 + 4Si-H fCH4 + 4Si-O (1)

Chart 1

Table 1. Reduction of CO2 with PhMe2SiH at Room Temperature

entry catal yst (mol %)a TOF (h-1) TONb yield (%)c time (h)

1 1/B(C6F5)3(0.45) 23 34 15 1.5

2 2/B(C6F5)3(0.42) 31 46 19 1.53 3/B(C6F5)3(0.44) 150 225 98 1.54d 3/[Ph3C][B(C6F5)4] (1.8) 0 1.55 B(C6F5)3(1.9) 0 e

a The Zr/B ratio )ca. 1. b TON and TOF are based on the zirconiumcomplex per Si-H bond. c Isolated yields of (PhMe2Si)2O. d Conversionof PhMe2SiH into Ph2Me2Si and Me2SiH2 was observed. e 3 days.

Published on Web 08/31/2006

12362 9 J. AM. CHEM. SOC. 2006,128, 12362-12363 10.1021/ja0647250 CCC: $33.50 2006 American Chemical Society

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